翻译文献-对实体bean的介绍

外文原文CHAPTER7INTRODUCTIONTOENTITYBEANSINCHAPTERS36,YOULEARNEDHOWTOCODESESSIONBEANSDISTRIBUTEDCOMPONENTSTHATREPRESENTBUSINESSPROCESSESBUTSESSIONBEANSAREONLYHALFOFWHATENTERPRISEJAVABEANSHASTOOFFERONEOFTHEKEYBENEFITSOFEJBISTHEPOWERTOCREATEENTITYBEANSENTITYBEANSAREPERSISTENTOBJECTSTHATCANBESTOREDINPERMANENTSTORAGETHISMEANSYOUCANMODELYOURBUSINESSSFUNDAMENTAL,UNDERLYINGDATAASENTITYBEANSWELLSEEEXACTLYWHATTHISMEANSINTHEPAGESTOCOMEINTHISCHAPTER,WELLCOVERTHESETOPICS_THEBASICCONCEPTSOFPERSISTENCE_ADEFINITIONOFENTITYBEANS,FROMAPROGRAMMERSPERSPECTIVE_THEFEATURESTHATENTITYBEANSHAVETOOFFER_HOWENTITYBEANSCOMPAREWITHSESSIONBEANS_ENTITYBEANPROGRAMMINGCONCEPTSENTITYBEANSAREANOPTIONALPARTOFENTERPRISEJAVABEANS10EVENIFYOUREJBCONTAINERISCOMPLIANTWITHEJB10,THEEJBCONTAINERMAYNOTBEABLETOHANDLEENTITYBEANSIFYOUNEEDENTITYBEANS,MAKESUREYOUCHOOSEACONTAINERCAPABLEOFDEPLOYINGTHEMEJB11,WHICHISPARTOFJAVA2PLATFORM,ENTERPRISEEDITIONJ2EE,MANDATESENTITYBEANSUPPORTTHISCHAPTERISRELATIVELYTHEORETICAL,ANDITISMEANTTOGIVEYOUADEEPFOUNDATIONINENTITYBEANPROGRAMMINGCONCEPTSFORTHOSEOFYOUWITHATRADITIONALPROCEDURALPROGRAMMINGBACKGROUND,ENTITYBEANSCANBEAVERYTOUGHTOPICTOGRASPYOUMAYNEEDTOREREADTHISCHAPTERAFEWTIMESTOREALLYUNDERSTANDHOWTHINGSWORKMAKESUREYOUVEREADANDUNDERSTOODTHEPREVIOUSCHAPTERSINTHISBOOKOURDISCUSSIONOFENTITYBEANSWILLBUILDONTHEKNOWLEDGEYOUVEACQUIREDSOFARWELLUSETHESECONCEPTSWITHHANDSONCODEINCHAPTERS8AND9PERSISTENCECONCEPTSBECAUSEENTITYBEANSAREPERSISTENTOBJECTS,OURDISCUSSIONBEGINSWITHAQUICKLOOKATJAVAOBJECTSERIALIZATIONWHENYOUWORKWITHJAVAOBJECTS,INMANYCASESYOUWOULDLIKETOCAPTURETHESTATEOFTHEOBJECTYOURECURRENTLYWORKINGWITHANDSAVEITTOAPERMANENTSTORAGEONEWAYTODOTHIS,ASCOVEREDINAPPENDIXA,ISTOUSEOBJECTSERIALIZATIONOBJECTSERIALIZATIONISANEASYWAYTOMARSHALANOBJECTGRAPHINTOACOMPACTREPRESENTATIONWHENYOUSERIALIZEANOBJECTGRAPH,YOUCONVERTTHEGRAPHINTOABYTESTREAMYOUCANTHENDOANYTHINGYOUWANTTOWITHTHATSTREAM,SUCHASPUSHTHEDATAOVERTHENETWORKWHICHISHOWJAVARMIPASSESPARAMETERSOVERTHENETWORK,ORYOUCANSAVETHESTREAMTOASTORAGE,SUCHASAFILESYSTEM,DATABASEORJNDITREEFORSOPHISTICATEDPERSISTENCE,HOWEVER,OBJECTSERIALIZATIONFALLSSHORTINMANYAREASFOREXAMPLE,LETSSAYWESTOREAMILLIONSERIALIZABLEBANKACCOUNTOBJECTSONTOAFILESYSTEMWEDOTHISBYCONVERTINGTHEOBJECTSTOTHEIRBITBLOBREPRESENTATIONANDTHENSTORINGTHEBYTESONDISKLETSSAYWETHENWANTTORETRIEVEALLBANKACCOUNTSTHATHAVEBALANCESOVER1000TODOTHISWITHSERIALIZATION,WEDHAVETOLOADEACHANDEVERYBANKACCOUNTSERIALIZEDBITBLOBFROMTHEDISK,CONSTRUCTTHECORRESPONDINGOBJECT,ANDTHENEXECUTEAMETHODQUERYONTHEOBJECTTODETERMINEIFTHEBALANCEISOVER1000WEMIGHTWANTTOPERFORMMOREADVANCEDQUERIESASWELL,SUCHASRETRIEVINGALLCHECKINGACCOUNTSTHATHAVEBEENINACTIVEFORSIXMONTHSTHEREISNOEFFICIENTWAYTODOTHISWITHOBJECTSERIALIZATIONINGENERAL,QUERYINGOBJECTSSTOREDUSINGOBJECTSERIALIZATIONISVERYEXPENSIVEANDCUMBERSOMESUBMITTINGQUERIESAGAINSTBUSINESSDATAISANABSOLUTENECESSITYFORLARGESCALEAPPLICATIONS,WHICHMAKESSIMPLEOBJECTSERIALIZATIONUNSUITABLEFORPERSISTENTSTORAGEWHILEOBJECTSERIALIZATIONHASITSPURPOSE,ITISBESTUSEDINRESTRICTEDDOMAINS,SUCHASFORNETWORKCOMMUNICATIONSANDSIMPLEPERSISTENCEFOREJB,WELLNEEDAMOREROBUSTPERSISTENCEMECHANISMTOADDRESSMORECOMPLEXQUERYINGOPERATIONSOBJECTRELATIONALMAPPINGANOTHERPOPULARWAYTOSTOREJAVAOBJECTSISTOUSEATRADITIONALRELATIONALDATABASE,SUCHASORACLEORMICROSOFTSQLSERVERRATHERTHANSERIALIZEEACHOBJECT,WECOULDDECOMPOSEEACHOBJECTINTOITSCONSTITUENTPARTSANDSTOREEACHPARTSEPARATELYFOREXAMPLE,FORABANKACCOUNTOBJECT,THEBANKACCOUNTNUMBERCOULDBESTOREDINONERELATIONALDATABASEROW,WHILETHEBANKACCOUNTBALANCECOULDBESTOREDINANOTHERROWWHENYOUSAVEYOURJAVAOBJECTS,YOUWOULDUSEJDBCORSQL/JTOMAPTHEOBJECTDATAINTOARELATIONALDATABASEYOUCOULDALSOSTORETHENAMEOFTHEJAVACLASSTHATTHISDATACORRESPONDSTO,SOTHATYOUKNOWWHICHCLASSTOINSTANTIATEWHENREADINGTHEOBJECTBACKFROMTHEDATABASEWHENYOUWANTTOLOADYOUROBJECTSFROMTHEDATABASE,YOUDFIRSTINSTANTIATEANOBJECTFROMTHATCLASS,READTHEDATAINFROMTHEDATABASE,ANDTHENPOPULATETHATOBJECTINSTANCESFIELDSWITHTHERELATIONALDATAREADINTHISISSHOWNINFIGURE71THISMAPPINGOFOBJECTSTORELATIONALDATABASESISATECHNOLOGYCALLEDOBJECTRELATIONALMAPPINGITISTHEACTOFCONVERTINGANDUNCONVERTINGINMEMORYOBJECTSTORELATIONALDATAANOBJECTRELATIONALO/RMAPPERCOULDUSEANYKINDOFUNDERLYINGDATABASESCHEMAFOREXAMPLE,ITCOULDTRANSLATEAJAVAOBJECTINTOASINGLERELATIONALRECORDITCOULDTHENRETRIEVETHATRECORDATALATERTIMETORECONSTRUCTANOBJECTINMEMORYFORYOUTOUSEOBJECTRELATIONALMAPPINGASAPERSISTENCEMECHANISMISAMUCHMORESOPHISTICATEDMECHANISMOFPERSISTINGOBJECTSTHANSIMPLEOBJECTSERIALIZATIONBYDECOMPOSINGYOURJAVAOBJECTSASRELATIONALDATA,YOUCANISSUEARBITRARYQUERIESFORINFORMATIONFOREXAMPLE,YOUCANSEARCHTHROUGHALLTHEDATABASERECORDSTHATHAVEANACCOUNTBALANCEENTRYGREATERTHAN1000ANDLOADONLYTHEOBJECTSTHATFULFILLTHISQUERYMOREADVANCEDQUERIESAREALSOPOSSIBLEMAPPINGOFOBJECTSTORELATIONALDATACANBEDONEINTWOWAYSYOUCANEITHERHANDCRAFTTHISMAPPINGINYOURCODEORUSEANOBJECTRELATIONALMAPPINGPRODUCTSUCHASOBJECTPEOPLESTOPLINKORSUNSJAVABLENDTOAUTOMATEORFACILITATETHISMAPPINGTODAY,MOSTUSERSHANDCRAFTTHEMAPPINGUSINGADATABASEACCESSAPISUCHASJDBCORSQL/JBECAUSETHECOSTOFDEVELOPINGANDMAINTAININGAN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询也是可行的。完成对象到关系数据的映射有两种方式。你既可以手动编程完成映射,也可以使用对象关系映射产品,例如OBJECTPEOPLE公司的TOPLINK或者SUN公司的JAVABLEND自动完成或促进完成这一映射。现在,大多数用户使用数据库存取API来完成手动映射,例如JDBC或者SQL/J。由于开发及维持对象关系映射层的成本很大,因此对象关系映射会因为它们的成熟而被人们采用。13对象数据库数据库管理系统是对于整个对象的持久性存储。在一个对象数据库中,你的对象是数据库的一等公民。这意味着没有O/R映射层你的JAVA对象本身作为整个对象保存。因此,你不需要编写关系数据库API的程序然而,你编写对象数据库的程序。这意味着你能避开对象/关系映射，从而简化了数据访问代码。大多数对象数据库（和对象/关系映射产品）通过使用对象查询语言OQL提供查询连续对象的工具。OQL是一个允许你通过任意特征查询对象属性的不错的高层次接口。它还添加了一个用来提取关系数据库查询的层。除了以OQL为基础的查询外,对象数据库还提供对象之间的关联。你可以定义一个银行账户对象和一个顾客对象之间的关联并且透明地在它们之间浏览。透明导航能够轻易实现对象模型的操作并且在与基于SQL的在关系数据库中完成相同工作的加入的比较中有出色表现。对象数据库，也有非常明确的性能和可扩展性。它们提供强大的完整性和安全性并未复杂的持久对象提供优良的存储。有一些应用在对象数据库中运行良好例如GEOSPATIAL或者CAD/CAM,这与关系数据库完全不适合。其他一些应用软件可以轻易映射关系数据库,例如大多数商业应用软件。对于简单的大容量的商业事务,关系数据库通常规模胜于对象数据库。OBJECTSTORE,VERSANT,和POET是一些提供对象数据库技术的供应商。不幸的是,对象数据库产品尚未完全受到业界欢迎。虽然它们在某些应用方面很有用,对象数据库由于缺少很多相关的工具而普遍受到限制,例如报告工具,调节工具和管理工具。现在我们通过持续机制使你的欲望兴奋起来,让我们来看看实体BEAN连续对象如何被使用在EJB多层环境中被使用的。2什么是实体BEAN在任何复杂的，面向对象的多层部署中，我们可以得出一个明确区分两类不同性质的成分调配应用逻辑元件。这些组件是完成一般任务的方法提供者。它们的任务包括以下内容计算一个命令的价值计算用户的信用卡计算逆矩阵注意这些组件所代表的行为它们是动词。这些信息非常适合用于处理业务流程。会话BEAN可以很好地建模这些应用逻辑组件。它们通常包含有趣的算法和逻辑来完成应用任务。会话BEAN为了一个用户来完成代表的工作。它们代表了包括任何工作流程的逻辑的用户会话。数据持久组件。这些都是知道如何使自身永久保存的对象可能用JAVA编写。它们使用一些持续机制,例如连续性,对于关系数据库的O/R映射,或者一个数据库。这些种类的对象表示了你想保存的简单的或者复杂的信息。这里的示例包括以下这些银行账户信息,例如账号和余额人力资源信息,例如姓名,部门和雇员的薪水领导跟踪信息,例如姓名,地址和你想随时知道的预期的顾客的电话号码。注意这些组件代表的人,地点,事务它们是名词。这些信息非常适合用于处理业务数据。你可能怀疑这些持久数据组件的必要性。为什么我们要以对象的方式处理商业数据,而不像处理原始数据